Small buffers introduced an issue that is exacerbated by the large fanout. A stream can
only be appended to forever once it has grabbed the initial io sized buffer. With small
buffers, we don't grab that at the beginning anymore and, before this patch, it is
grabbed when the stream first needs it. This means when one stream needs it, another
stream could have already grabbed it (meaning this stream is pinned with multiple
buffers).
This patch has all the streams grab an IO buffer as soon as the first stream needs an
io buffer. This guarantees that all streams get 1 before any get 2.
Change-Id: I1be1219fc5f1fa3ceedd4d5e76ae056c8bb8ff3d